EP0853995B1 - Injection moulding composition containing metal oxide for making metal shapes - Google Patents

Abstract

Moulding material containing 20-50 vol.% of a powder of one or more metal oxides, optionally together with metal carbides and/or nitrides which are not reduced by hydrogen, in a fluid binder, ≥ 65 vol.% of the powder having NOTGREATER 0.5 mu m particle size, with the remainder having NOTGREATER 1 mu m particle size, and ≥ 90 vol.% of the powder consisting of metal oxides, which are reduced by hydrogen. Preferably, the fluid binder is an organic polymer. Also claimed is the use of hydrogen-reducible metal oxides of particle size NOTGREATER 0.5 mu m for producing injection moulding materials. Further claimed is a method of producing metal mouldings by injection moulding the above moulding material, removing the binder and then reducing and sintering the resulting moulding in the presence of hydrogen.

Description

The invention relates to molding compositions containing metal oxides, in particular Injection molding compositions which are suitable for the production of metal moldings, as well as processes for the production of metal moldings.

In the fabrication of small complex metal moldings by the powder injection molding metal powder having powder diameter of 2 to 40 μ m are mixed with a flowable binder and sprayed to this mixture, as is customary in the processing of plastics by means of injection molding machines at pressures up to 2000 bar into a mold. The injection molding compound solidifies in the mold, usually because the mold has a lower surface temperature than the injected compound and the binder in the mold is cooled to a temperature below the glass transition temperature or melting temperature.

Then the mold is opened and the molded part is removed. The binder is then removed from the molding thus formed, and the molding should not be deformed. The binder can be removed in various ways. It is possible to thermally decompose the mostly organic binder by carefully increasing the temperature over a longer period of time and thus to remove it. The binder can also be constructed so that it is partially soluble in a solvent and this portion can be extracted with the solvent. The further binder portion is then thermally decomposed, which can be done more quickly than in the first variant, because after the soluble binder portion has been extracted, an open-pored body is already present and the thermal decomposition therefore does not create any internal pressure which could destroy the molding. The binder is most elegantly removed using a catalytic process, the binder used being, for example, a polyacetal which is directly depolymerized below its melting temperature under the influence of gaseous acids without the formation of a liquid phase to form gaseous formaldehyde. This process runs from the outside to the inside of the molding walls, which means that the entire gas exchange can also only take place in the already porous parts by volume, and likewise no disadvantageous internal pressure can be built up. This process has the further advantage that the debinding process takes place below the melting point of the binder, and the molding does not disadvantageously change its dimensions. Moldings which are very true to dimension are thus obtained. The deviation of the linear dimensions from the target dimension is a maximum of +/- 0.3%, often less. However, the roughness depths of the molded parts are essentially determined by the powder size used, so that roughness depths R _Z of 1 μm are not undercut. For the production of parts with smaller roughness metal powder would be of smaller diameter than 2 μ m necessary. However, the production of such metal powders is extremely expensive or there are considerable difficulties in handling such fine metal powders. The ratio of surface area to volume increases with decreasing particle size, which makes the metal powders more and more chemically reactive. Base metals, such as iron, cobalt, zinc or nickel, become pyrophoric and cannot be processed in air.

In addition, in the production of metal powders, the spraying of metal melts means that particle sizes of 5 μm are hardly exceeded. Often, the metal powders cannot be further crushed by grinding because they are too ductile.

US-A-4 415 528 relates to a process for the production of metal moldings by Injection molding. For example, nickel powder and iron powder are used Chromium oxide powder with a particle size of 0.5 µm mixed.

However, there has been a demand for finer molding compositions for the production of metal moldings since new techniques have succeeded in producing ever finer mold inserts for the injection molding process. The LIGA process is used, for example, to produce tool inserts that are used to produce parts by injection molding that have dimensions in the μm range and roughness in the nanometer range.

In the LIGA process, a light-sensitive polymer layer is placed on a base plate, a so-called photoresist, applied and through a mask, which contains the structures to be produced in cross section. The portions of the polymer layer exposed through the mask become soluble and can therefore be washed out. The trenches that were created electroplated through a metal layer, after which the remaining one Photoresist is dissolved. The metal structure thus obtained can be used as a mold can be used for an injection mold.

The object of the invention is to provide molding compositions or injection molding compositions for the production of metal moldings that have a property profile have their use in very fine mold inserts, for example allowed from the LIGA process. The shaped bodies obtained in this way are said to be in Fineness and surface quality of those produced using the LIGA process Conform shapes.

The object is achieved by molding compositions according to claim 9, containing in a flowable binder 20 to 50 vol .-%, based on the total volume of the molding composition, of a powder of one or more metal oxides and, if appropriate, metal carbides and / or metal nitrides which cannot be reduced with hydrogen, where at least 65% by volume of the powder have a particle size of at most 0.5 μm and the rest of the powder have a maximum particle size of 1 μm , and at least 90% by volume of the powder consist of metal oxides which can be reduced with hydrogen.

The present inventors have found that you can use instead of the large-grained, poorly accessible and difficult to handle metal powder, metal oxide powder with particle sizes below 1 μ m for the production of the molding materials. In this process, the molding compound or injection molding compound is shaped into a molding, the molding is debindered and sintered with a reduction in the metal oxides in a reducing atmosphere containing hydrogen.

A powder is used which has at least 65% by volume of a particle size of at most 0.5 μm , the rest of the powder having a particle size of at most 1 μm . Particularly preferably, at least 80% by volume of the powder have a particle size of at most 0.5 μm . At least 90% by volume of the powder consists of hydrogen-reducible metal oxides, the remaining portion of the powder consisting of hydrogen-reducible metal oxides, metal carbides and / or metal nitrides.

Suitable metal oxides are those which are reducible with hydrogen and are sinterable, so that they can be produced from them by heating under a hydrogen atmosphere or in the presence of hydrogen. Examples of metals whose oxides can be used can be found in groups VIB, VIII, IB, IIB, IVA of the periodic table. Examples of suitable metal oxides are Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , NiO, CoO, Co ₃ O ₄ , CuO, Cu ₂ O, Ag ₂ O, WO ₃ , MoO ₃ , SnO, SnO ₂ , CdO, PbO, Pb ₃ O ₄ , PbO ₂ , Cr ₂ O ₃ . The lower oxides are preferably used, such as Cu ₂ O instead of CuO and PbO instead of PbO ₂ , since the higher oxides are oxidizing agents which, under certain conditions, can react, for example, with organic binders. The oxides can be used individually or as mixtures. For example, pure iron moldings or pure copper moldings can be obtained. When using mixtures of the oxides, alloys and doped metals are accessible, for example. For example, steel parts are produced from iron oxide / nickel oxide / molybdenum oxide mixtures and bronzes from copper oxide / tin oxide mixtures which may also contain zinc, nickel or lead oxide. Particularly preferred metal oxides are iron oxide, nickel oxide and / or molybdenum oxide.

The metal oxides used in this invention having a particle size of not more than 1 μ m, preferably not more than 0.5 μ m, can be prepared by different methods, preferably prepared by chemical reactions. The hydroxides, oxide hydrates, carbonates or oxalates, for example, can be precipitated from solutions of metal salts, the particles possibly being obtained in very fine particles in the presence of dispersants. The precipitates are separated and brought to the highest possible purity by washing. The precipitated particles are dried by heating and converted to the metal oxides at elevated temperatures.

It is also possible to get very finely divided metal oxides directly in one step. For example, by burning iron pentacarbonyl with oxygen, extremely fine, spherical iron oxide particles with specific surfaces of up to 200 m ² / g are obtained.

The metal oxides or at least 65% by volume of the powder used according to the invention preferably have a BET surface area of at least 5, preferably at least 7 m ² / g.

In addition to the metal oxides that can be reduced with hydrogen, other metal compounds that cannot be reduced during sintering, such as metal oxides, metal carbides or metal nitrides that cannot be reduced with hydrogen, can also be present. Examples of oxides are ZrO ₂ , Al ₂ O ₃ and TiO ₂ . Examples of carbides are SiC, WC or TiC. An example of a nitride is TiN.

The one used in the molding compositions according to the invention Powder has at least 90% by volume, particularly preferably at least 95% by volume, based on the powder, on hydrogen-reducible metal oxides. If metal oxides, metal carbides and / or not reducible with hydrogen Metal nitrides are used, they are preferably in amounts of 1 to 10, particularly preferably 2 to 5% by volume, based on the powder, in front.

The powder used according to the invention is present in the molding compositions in quantities from 20 to 50% by volume, preferably 25 to 45% by volume, particularly preferably 30 to 40 vol .-%, based on the total volume of the molding compound.

The powder used in the molding compositions according to the invention is distributed in a flowable binder. If necessary, additional a dispersant can be used. According to a preferred embodiment The invention consists of the molding composition from that described above Powder, a flowable binder and optionally a dispersant.

According to a further embodiment of the invention, the molding compound has In addition to these components, other components as described below are described.

The total volume of all ingredients of the molding compound results in each Fall 100 vol%.

All binders which are suitable for use in the powder injection molding process can be used as flowable binders. They are preferably flowable at the processing temperature so that they can be injection molded into molds. For example, the binders described above in the prior art can be used. It is therefore possible to use binders which are thermally decomposed and thus removed, binder mixtures, some of which can be extracted with solvents and the other part can be thermally decomposed, or binders which are used, for example, in the form of a polyacetal which is below its melting temperature below the influence of gaseous acids can be depolymerized directly to gaseous products without the formation of a liquid phase. Suitable binders are known to the person skilled in the art. The flowable binder preferably contains an organic polymer. A polyoxymethylene copolymer is used in the molding composition according to claim 9, as described, for example, in EP-A-0 444 475, EP-A-0 446 708 and EP-A-0 444 475. It is preferably a polyoxymethylene copolymer which contains 0.5 to 10, preferably 1 to 5 mol% of butanediol formal as comonomer. Polybutanediol formal can be used as an additional binder.
A mixture of 75 to 89% by weight of polyoxymethylene copolymer which contains 2 mol% of butanediol formal as a comonomer and has a melt index of about 45 g / 10 min at 190 ° C. and 2.16 kg coating weight is particularly preferred, and 11 to 25 wt .-% polybutanediol formal with a molecular weight M _n of about 20,000 used.

All dispersants used for Dispersion of metal oxide particles of the specified particle size in Binders are suitable. A suitable class of substances for the dispersants are alkoxylated fatty alcohols or alkoxylated fatty acid amides.

Other suitable ingredients of the molding compositions are those during processing processing stabilizers used by polyoxymethylene.

The molding compositions according to the invention are for injection molding usable from metal moldings. It is used to manufacture the Molding the organic and inorganic components in suitable Mixers mixed. This is preferably done in a kneading device while melting the flowable binder. After solidification the molding compositions are preferably granulated. You can be injection molded by known methods, preferably at melt temperatures from 170 to 200 ° C. The shape used preferably has a temperature of 120 to 140 ° C.

The binder is then removed from the moldings thus obtained. Depending on the binder used, this can be done by slowly heating, Treat with a solvent and then heat or Treat with an acid and heat up. This is preferably done Debinder simultaneously with heating to reduce and sinter the Shaped. The molding in the presence of hydrogen is preferred under hydrogen atmosphere, at a rate of 1 to 20 ° C / min, preferably 2 to 10 ° C / min to the material-specific sintering temperature heated, 1 to 20, preferably 2 to 10 hours at Leave the sintering temperature and then cool down. During the slow The binder is removed by heating. The one used for reduction Hydrogen preferably has a maximum dew point of -10 ° C, particularly preferably from less than -40 ° C. The dew point will be chosen so that a reduction under the Reaction conditions is possible.

To reduce Cr ₂ O ₃ , for example, an extremely dry hydrogen with a dew point of less than -40 ° C is required. The reduction is carried out at temperatures above 1500 ° C., particularly preferably above 1600 ° C. When sintering chromium-containing alloys, the alloy components often sinter at 1200 to 1300 ° C, while when using Cr ₂ O ₃ this can still remain in the molding unreduced. In the production of, for example, stainless steels with a chromium content of approximately 13 to 20% by weight, the chromium content is therefore preferably used as ferrochrome with a particle size of the particles of at most 1 μm . The volume fraction of the ferrochrome is preferably less than 35% by volume. It is thus possible to produce stainless steels alloyed with chromium and, if appropriate, nickel and molybdenum, without fear that non-reduced Cr ₂ O ₃ remains in the otherwise already sintered molding.

The invention also relates to a method for producing molded metal bodies by injection molding a molding compound as described above is, in a form, removal of the binder from the thus obtained Molding and reducing and sintering the debindered molding into one Shaped metal body in the presence of hydrogen. The removal takes place of the binder preferably thermally in one step with the reducing and sintering by heating the molding to the sintering temperature in Presence of hydrogen.

With reducing sintering, the moldings shrink up to 5 times, based on volume or up to half based on linear Dimensions. This high shrinkage is just for making very Small structures are an advantage because the injection mold is about the Factor 2 can be made larger in every dimension and therefore very large fine details can be formed. The dimensional tolerances of the sintered Moldings are preferred despite the high absolute shrinkage maximum +/- 0.3%, particularly preferably +/- 0.15%.

The surface roughness R _Z is preferably less than 1 μ m, R _a less than 0.2 μ m, measured according to DIN 4768 or DIN 4768/1.

The invention is explained in more detail below with the aid of examples.

Examples

The injection molding compounds listed in the examples below were Manufactured according to a uniform procedure, thermally debindered and when the material is adequate Sintering reducing temperatures under hydrogen.

A thermoplastic polyoxymethylene copolymer which contained 2 mol% of butanediol formal as a comonomer and had a melt index of about 45 g / 10 min at 190 ° C. and 2.16 kg coating weight was used as the flowable binder. Polybutanediol formal with a molecular weight M _n of approximately 20,000 was used as an additional binder. Solsperse® 17000 from ICI was used as the dispersant for dispersing the inorganic powders. The amounts are given in the table below.

The organic and inorganic components of the molding compound were in a paddle mixer of 1 l useful content melted at 190 ° C and for Kneaded for 90 min. The paddle mixer was then cooled and the mass solidified and granulated in the rotating kneader. The so obtained Injection molding compounds were at 180 ° C melt temperature in a to 130 ° C tempered mold for a bending rod with the dimensions 1.5 x 6 x 50 mm injected.

The bending bars produced in this way were heated up in a tube furnace under a hydrogen atmosphere (hydrogen with a dew point around -10 ° C.) at a rate of 2 ° C./min to the specified material-specific sintering temperature and left at the sintering temperature for 2 hours. The oven was then cooled. During the slow heating process, the polyoxymethylene and the polybutanediol formal depolymerized in the temperature range from 220 to 300 ° C. without the formation of cracks in the thin-walled bending rod. The flexural bars were supported on a powder bed of alumina powder with about 5 μ m particle size, in order to facilitate the shrinking.

All of the molding compositions listed in the examples resulted in flawless, crack-free moldings, although the volume shrinkage is sometimes 80% scam.

The surface roughness values obtained with a polished injection mold were in any case less than 1 μm for R _Z and less than 0.2 μm for R _a .

Claims (10)

1. The use of a molding composition containing, in a flowable binder, from 20 to 50% by vol., based on the total volume of the molding composition, of a powder comprising one or more metal oxides and, if desired, metal carbides and/or metal nitrides which cannot be reduced using hydrogen, where at least 65% by vol. of the powder has a maximum particle size of 0.5 µm and the remainder of the powder has a maximum particle size of 1 µm, and at least 90% by vol. of the powder comprises metal oxides which can be reduced using hydrogen, as injection-molding composition for the production of metal moldings.
2. The use as claimed in claim 1, wherein at least 65% by vol. of the powder of the molding composition has a BET surface area of at least 5 m²/g.
3. The use as claimed in claim 1 or 2, wherein the flowable binder of the molding composition contains an organic polymer.
4. The use as claimed in any one of claims 1 to 3, wherein the molding composition contains a dispersant for the powder.
5. The use as claimed in any one of claims 1 to 4, wherein, in the molding composition, the metal oxides which can be reduced using hydrogen are Fe₂O₃, FeO, Fe₃O₄, NiO, CoO, Co₃O₄, CuO, Cu₂O, Ag₂O, Bi₂O₃, WO₃, MoO₃, SnO, SnO₂, CdO, PbO, Pb₃O₄, PbO₂ or Cr₂O₃, or mixtures thereof.
6. The use as claimed in any one of claims 1 to 5, wherein the powder in the molding composition contains from 1 to 10% by vol. of metal oxides, metal carbides or metal nitrides which cannot be reduced using hydrogen, or mixtures thereof, having a maximum particle size of 0.5 µm.
7. A process for the production of metal moldings by injection molding a molding composition as defined in any one of claims 1 to 6 in a mold, removing the binder from the resultant molding, and reducing and sintering the debindered molding in the presence of hydrogen to give a metal molding.
8. A process as claimed in claim 7, wherein the removal of the binder is carried out thermally in a single step together with the reduction and sintering by heating the molding to the sintering temperature in the presence of hydrogen.
9. A molding composition containing, in a flowable binder comprising a polyoxymethylene copolymer and, if desired, additionally butanediol formal, from 20 to 50% by vol., based on the total volume of the molding composition, of a powder comprising one or more metal oxides and, if desired, metal carbides and/or metal nitrides which cannot be reduced using hydrogen, where at least 65% by vol. of the powder has a maximum particle size of 0.5 µm and the remainder of the powder has a maximum particle size of 1 µm, and at least 90% by vol. of the powder comprises metal oxides which can be reduced using hydrogen.
10. A molding composition as claimed in claim 9, wherein the polyoxymethylene copolymer contains from 0.5 to 10 mol % of butanediol formal as comonomer.